The present invention relates to apparatus for detecting the presence of voice signals on a telephone line or the like, and more particularly, is directed to a circuit for discriminating the existence of a voice signal on a telephone line from other signals, such as busy-tones, dial-tones, etc., normally occurring on a telephone line.
It is known in the prior art to provide signal discrimination circuits for discriminating one type of signal from other types of signals. In particular, such signal discrimination circuits are often used for detecting the existence of voice signals on a telephone line. Such apparatus has utility in automatic telephone response systems for restarting a timing mechanism in the automatic system. Such signal discriminating circuits also have other utilities as will be well understood by anyone of ordinary skill in the art.
The conventional method of discriminating voice signals from other common signals occurring on a telephone line is based upon a statistical feature of voice signals. That is, a voice signal will normally have a pause of 0.6 seconds or longer every few seconds. Thus, the conventional signal discriminator operates to detect the existence of such pauses. Problems exist, however, in adapting such discriminators for use with telephone lines in several different countries. Whereas a busy-tone signal in some countries will have pauses shorter than 0.6 seconds, in other countries the busy-tones will have pauses which are equal to or longer than 0.6 seconds. Obviously, the signal detector can be altered so as to detect longer pauses, thereby being insensitive to the occurrence of the busy-tone, but such adjustment must be made in a factory. Furthermore, if the discriminator is adjusted to detect pauses greater than 1 second, the effectiveness of the signal discriminator will be severely hampered.